Conventionally, techniques are known for detachably mounting, on a helmet, a head-mounted display assembly provided with a display that displays images (see, for example, Patent Document 1).
Disclosed in Patent Document 1 is a technique that in order to make a head-mounted display assembly commonly usable among a plurality of helmets, the head-mounted display assembly is mounted on each helmet by a holder, which has resiliency, while holding the outer side of the helmet fixedly in each of opposite directions.